TRACHEATA 391 



produced into a process which helps to surround the mouth 

 and form a biting organ. In the female of L. polyphemus, the 

 first five of these appendages are chelate, but in the male the 

 first is enlarged but is not chelate ; in both sexes the sixth 

 appendage terminates in a number of elongated flattened 

 plates, and this limb is used in the burrowing or digging 

 operations in which the animal delights. All these append- 

 ages are borne on the prosoma ; the seventh appendage, or the 

 first mesosomatic, consists of a semicircular plate-like structure 

 hinged on to the body and bearing on its posterior face the 

 two genital pores. This genital plate or operculum folds over 

 and almost covers the five succeeding appendages, which are also 

 plate-like, but, like the former, exhibit traces of a double origin. 

 Each of these, from the eighth to the twelfth, carries a pair of 

 respiratory organs, in the form of branchiae composed .of a great 

 number of thin plates like the leaves of a book. Behind the 

 twelfth is the unsegmented metasoma, which bears no appendages. 



An internal skeleton or endosfernite, in the form of a plate 

 of fibro- cartilage, lies between the alimentary canal and the 

 elongated nerve collar. It is not connected in any way with 

 the exoskeleton, but gives origin to a number of muscles. A 

 somewhat similar structure is found in Apus and in some 

 other Crustacea. 



The alimentary canal, as is usual with the Arthropoda, 

 excepting the Insects, runs in a median line from mouth to 

 anus without convolutions. The mouth is situated some dis- 

 tance behind the anterior end of the body; it leads into a 

 suctorial pharynx, which is lined by chitinous ridges. The 

 pharynx runs forward and widens into a stomach, which turns 

 back, curves posteriorly, and is separated by a valve from the 

 mid-gut ; the latter is the absorbent part of the alimentary tract. 

 It extends through the body, and terminates in a short procto- 

 daeum with folded walls. The only gland which opens into the 

 alimentary canal is a large yellow organ, the so-called liver, 

 which communicates with the mid-gut by two pairs of ducts. 



The vascular system is very complete, and, unlike many 

 Arthropods, the arteries do not end in irregular lacunar spaces, 

 but are connected with the veins by a definite capillary system. 

 The heart consists of an elongated tube, partially divided into 



